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Journal of Solution Chemistry

, Volume 47, Issue 8, pp 1373–1394 | Cite as

A Short Review of the Separation of Iridium and Rhodium from Hydrochloric Acid Solutions by Solvent Extraction

  • Minh Nhan Le
  • Man Seung Lee
  • Gamini Senanayake
Article
  • 160 Downloads

Abstract

Iridium and rhodium are among the platinum group metals. The properties, production processes, and aqueous chemistry of both metals are reviewed. The separation of Ir(IV) and Rh(III) from hydrochloric acid solution is dependent on the characteristics of the solvent extraction systems. In most of the extraction conditions, Ir(IV) is selectively extracted over Rh(III) by either amines or neutral extractants. Rh(I) can be selectively extracted over Ir(III) by neutral extractants after Rh(III) is reduced in the presence of a reducing agent. The separation of these two metals using cationic extractants has also been reported. Although selective extraction of one metal over the other is possible, more efficient solvent extraction systems need to be developed.

Keywords

Separation Solvent extraction Iridium Rhodium Aqueous chemistry 

1 Introduction

Platinum group metals (PGMs) are a group of precious metals comprising six elements: ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt). The rapid development of industries results in the depletion of the resources containing these metals. Therefore, it is necessary to find an effective separation process for recovering these metals with high purity from either low-grade ores or secondary resources [1]. In platinum group metals, iridium and rhodium are grouped in the same column of the periodic table. They are widely used in making precise alloys, apparatus, corrosion-resistant chemical wares, plug electrodes, crucibles for high temperature reactions and extrusion dies for high melting glasses, automobile and chemical industries owing to their specific physical and chemical properties [2, 3, 4, 5, 6]. Since they have similar chemical properties, the separation and purification of iridium and rhodium is very difficult.

Table 1 lists some important properties of iridium and rhodium [7, 8, 9], indicating the similarity in the physical and chemical properties of the two metals. Either sodium peroxide or sodium bisulfate fusion treatment of iridium and rhodium metals converts them into soluble forms that are then dissolved in aqua regia [7]. Metallic rhodium is harder, tougher and has a higher melting point than platinum or palladium. However, it is softer, more ductile and less brittle than metallic iridium [9]. In the earth’s crust, rhodium and iridium are the 79th and the 84th abundant elements and their concentrations are lower than 0.5 ng·g−1 for Rh and 0.3–5 ng·g−1 for Ir [10, 11]. The common minerals containing iridium are the alloys of osmium and iridium known as osmiridium (syserkskite), iridosmium (nevyanskite) or ruthenosmiridium. Rhodium is associated with iridium and rhodium as arsenides such as hollingworthite (Ir: 1.4–3.1%, Rh: 24.6–30.8%) and irsarite (Ir: 23%, Rh: 7.2%) [12, 13, 14].
Table 1

The main properties of iridium and rhodium [7, 8, 9]

Property

Iridium

Rhodium

Atomic number

77

45

Atomic weight (g·mol−1)

192.2

102.91

Electron ground state

[Xe] 4f145d76s2

[Kr] 4d85s1

Density at 25 °C (g·cm−3)

22.42

12.41

Crystal structure

Face-centered cubic

Face-centered cubic

Melting point (°C)

2443

1966

Boiling point (°C)

4500

3727

Electronegativity, Pauling scale

2.20

2.28

Metallic radius (Å)

1.36

1.34

Resistivity (microhm-cm at 20 °C)

4.71

4.33

Heat of fusion (kJ·mol−1)

27.6

21.8

First ionization potential (eV)

8.7

7.7

Standard redox potential M/M2+ (V)

1.1

0.6

Thermal neutron cross-section (barns atom)

440

156

Although some review papers have been published on the separation of platinum group metals [15, 16, 17], there are few review papers on the separation of Ir(IV) and Rh(III) by solvent extraction. This paper reviews the production processes and aqueous chemistry of iridium and rhodium, and also highlights the selective extraction of one metal over the other according to the extraction systems.

2 Production Processes of Pure Iridium and Rhodium Metals in Industry

In the recycling industry, there are three main categories of refining materials. Primary materials like gold and silver ores and platinum group metal concentrates originating from the mining industry. Secondary materials include chemical, petrochemical, automotive catalysts, and sweeps or bullion-type materials from industrial applications. The third material consists of waste from other precious metal refining plants, such as insoluble residues from wet chemical systems, PGM sweeps from Ag/Au refiners, and Ir/Ru/Rh-concentrates [18].

Initially, PGMs are dissolved in chloride solution in the presence of an oxidizing agent. Metal ions are separated through several steps, such as precipitation, dissolution under the control of chemical methods [18, 19]. Metal refining plants have improved their processes to improve the separation efficiency by employing either solvent extraction or ion exchange. A typical flowsheet for the International Nickel Company’s process for the recovery of the platinum metals at the Acton refinery is shown in Fig. 1 [8, 20, 21]. The concentrates of platinum group metals are first dissolved in aqua regia, which dissolves most of the palladium, platinum, and gold and leaves a residue containing ruthenium, rhodium, iridium and silver chloride. Most of the precious metals in the residues are converted to either their chloride or ammonium salts by adding inorganic substances at high temperature and then they are recovered. In this process, iridium and rhodium are recovered by precipitation.
Fig. 1

Flow sheet for the recovery of iridium and rhodium from PGM

In solvent extraction processes, they remain in the raffinate after the extraction of Pt(IV) and Pd(II) [20]. The precipitate of Rh(III) hydroxide is dissolved in hydrochloric acid and treated with sodium nitrite to convert the rhodium to sodium hexanitrorhodate(III). After filtration, ammonium chloride is added to precipitate (NH4)3[Rh(NO2)6] which is redissolved in hydrochloric acid to convert the rhodium to the RhCl 6 3− anion. Formic acid is added to reduce the rhodium from the trivalent state to the metallic state. Then, it is heated in a hydrogen atmosphere at 1000 °C to give a pure rhodium sponge. In the case of iridium, Ir(IV) oxide from the potassium hydroxide-potassium nitrate fusion is dissolved in aqua regia. Addition of ammonium chloride yields a precipitate of ammonium hexachloroiridate(IV), (NH4)2IrCl6. The impure salts are treated by repeated crystallization or dissolved in ammonium sulfide solution to remove them as their sulfides. In the final step, the precipitate of pure (NH4)2IrCl6 is heated under hydrogen gas to produce pure iridium powder at 1000 °C. These processes are complicated and consume a large amount of energy and thus the development of a simple process to recover these metals from industrial wastes is necessary.

The separation of iridium and rhodium is an essential step for the production of iridium and rhodium metals. Common methods, such as precipitation [22, 23, 24], solvent extraction [25, 26, 27, 28, 29, 30, 31, 32, 33], ion exchange [34, 35, 36, 37, 38] and adsorption [39] are employed. Among these, solvent extraction is the most common method for the recovery of iridium and rhodium.

In solvent extraction, the distribution coefficient (D) is defined as the ratio of the equilibrium concentration of a metal ion in the organic phase ([X]o) to that in the aqueous phase ([X]a), as shown in Eq. 1. Table 2 lists some values of the distribution coefficients for the separation of iridium and rhodium by solvent extraction.
Table 2

A list of some distribution coefficients of iridium(IV) and rhodium(III) from hydrochloric acid solution by various extractants

Organic

Aqueous

D Ir

D Rh

Ref.

Polyurethanea

[Ir] = [Rh] = 100 mg·L−1, [HCl] = 2 mol·L−1

208.9

11,749

[63]

TBPb

[Ir] = 177 mg·L−1, [Rh] = 247 mg·L−1, [HCl] = 5 mol·L−1

3.53

0.29

[82]

Diantipyryl Propylmethaneb

[Ir] = 0.8 g·L−1, [Rh] = 1 g·L−1, [HCl] = 1 mol·L−1

125

0.35

[65]

Cyanex 921b

[Ir] = [Rh] = 50 mg·L−1, [HCl] = 1 mol·L−1

1.81

0.004

[61]

TOAc

[Ir] = 1.4 g·L−1, [Rh] = 1.5 g·L−1, [HCl] = 6 mol·L−1

89.90

0.06

[60]

TOAc

[Ir] = 200 mg·L−1, [Rh] = 120 mg·L−1, [HCl] = 1 mol·L−1

46.54

0.56

[59]

TEHAc

[Ir] = 200 mg·L−1, [Rh] = 120 mg·L−1, [HCl] = 1 mol·L−1

19.15

0.25

[59]

Alamine 336c

[Ir] = [Rh] = 100 mg·L−1, [HCl] = 1 mol·L−1

0.38

0.04

[62]

Alamine 336c

[Ir] = [Rh] = 50 mg·L−1, [HCl] = 1 mol·L−1

6.40

0.37

[61]

Alamine 330c

[Ir] = [Rh] = 50 mg·L−1, [HCl] = 1 mol·L−1

6.80

0.45

[61]

Alamine 308c

[Ir] = 200 mg·L−1, [Rh] = 120 mg·L−1, [HCl] = 1 mol·L−1

35.36

0.52

[59]

Aliquat 336c

[Ir] = 200 mg·L−1, [Rh] = 120 mg·L−1, [HCl] = 1 mol·L−1

29.70

0.36

[59]

Aliquat 336c + LIX 54a

[Ir] = [Rh]h = 50 mg·L−1, [HCl] = 1 mol·L−1

1.1

0.031

[61]

DIr, DRh are distribution coefficients of iridium and rhodium, respectively

aAcidic extractants

bNeutral extractants

cAmine-based extractants

$$ D = [{\text{X}}]_{\text{o}} /[{\text{X}}]_{\text{a}} $$
(1)

The principle of their separation by solvent extraction is based on the difference in the distribution coefficients of iridium and rhodium between aqueous and organic phases [40, 41]. The extraction and separation of metals depends on the speciation of the metal ions in aqueous solutions [42]. Therefore, the solution chemistry of iridium and rhodium in the aqueous phase needs to be understood to develop new processes. The species of iridium and rhodium in an aqueous solution can be classified as cationic, neutral, and anionic, which depend on solution pH, metal ion concentration, and the nature of the ligands in the solution. The separation of Ir(IV) and Rh(III) using solvent extraction makes use of the different types of extractable species formed between the metal complexes and the extractants. A brief review of the solution chemistry for the separation of Ir(IV) and Rh(III) from hydrochloric acid solutions reported in the literature will be presented in the next section which could promote the development of processes for efficient separation of iridium and rhodium using solvent extraction.

3 The Aqueous Chemistry of Iridium and Rhodium

3.1 Oxidation States

The main oxidation states of rhodium are + 1 and + 3, with the + 3 complexes usually being predominant in chloride system. Other oxidation states are rare. Rhodium(III) forms octahedral complexes with halide anions or with oxygen-containing ligands. In general, it is difficult to extract highly charged octahedral complexes like RhCl 6 3− due to the difficulty in packing three organic cationic molecules around a single anion due to steric effects [43]. In the case of iridium, the important valence states are + 3 and + 4 [44]. Iridium forms a number of stable compounds with halogen and oxygen donors. The other valence states are limited to a few compounds. The oxidation states of iridium and rhodium are listed in Table 3 [8, 43, 44].
Table 3

Examples of compounds in the various oxidation states of iridium and rhodium

Oxidation state

Coordination number

3

4

5

6

Ir(–II)

Ir(NO)2PPh3

   

Ir(–I), Rh(–I)

 

K[Ir(PF3)4], [Rh(CO)4]

[Rh(NO)2Cl]n

 

lr(0), Rh(0)

 

Ir4(CO)12

 

Rh4(CO)12

Ir(I), Rh(I)

 

Ir(CO)Cl(PPh3)2,

[Rh(CO)2Cl]2

HIr(CO)(PPh3)3,

HRh(PF3)4

 

Ir(II), Rh(II)

 

Ir(CO)2Cl2,

[Rh(C4N2S2)2]2−

[Ir(CO)2I3],

[Rh(OAc)2]2

 

Ir(III), Rh(III)

  

H3Ir(AsPh3)2

[Rh(H2O)6]3+, [IrCl6]3−

Ir(IV), Rh(IV)

   

Cs2[RhCl6] [IrCl6]2−

Ir(V), Rh(V)

   

[IrF6], [RhF5]4

Ir(VI), Rh(VI)

   

IrF6, RhF6

3.2 Iridium(III)/(IV) Species

In solution, iridium(IV) can be prepared by adding an alkali metal chloride to a suspension of hydrous IrO2 in aqueous HCl. Another way is to use a mixture of iridium powder and alkali metal chloride to prepare iridium(IV) by chlorination. In hydrochloric acid solution, stable chloro-complexes of Ir(IV), such as IrCl 5 and IrCl 6 2− , are formed [45, 46].

Among the Ir(IV) species, IrCl4 and HIrCl5 can be extracted by neutral extractants like Cyanex 923 [47]. Solution pH affects the distribution of Ir(IV) species in chloride solutions: \( {\text{IrCl}}_{3} ( {\text{H}}_{ 2} {\text{O)}}_{3}^{ + } \), IrCl4(H2O)2, \( {\text{IrCl}}_{5} ( {\text{H}}_{ 2} {\text{O)}}^{ - } \), \( {\text{IrCl}}_{ 4} ( {\text{OH)}}_{ 2}^{2 - } \) and \( {\text{IrCl}}_{2} ( {\text{OH)}}_{4}^{2 - } \). When the solution pH is below 2, Ir(IV) predominantly exists as \( {\text{IrCl}}_{6}^{2 - } \). In the pH range from 2 to 7, the fractions of \( {\text{IrCl}}_{ 4} ( {\text{OH)}}_{2}^{2 - } \) and \( {\text{IrCl}}_{2} ( {\text{OH)}}_{4}^{2 - } \) increase with increasing pH. Negatively charged \( {\text{IrCl}}_{2} ( {\text{OH)}}_{ 4}^{{ 2 { - }}} \) and neutral IrO·2H2O exist in the pH range from 7 to 12 [48]. The fast exchange of iridium oxidation state between \( {\text{IrCl}}_{6}^{2 - } \) and \( {\text{IrCl}}_{6}^{3 - } \) has been reported in dilute acid solution [8]. The effect of acidity on the speciation of Ir(IV) can be identified using capillary zone electrophoresis and spectrophotometry measurements [49].

In alkaline solution, the dark red-brown \( {\text{IrCl}}_{6}^{2 - } \) is rather unstable, undergoing spontaneous reduction within minutes to form pale yellow-green \( {\text{IrCl}}_{6}^{3 - } \):
$$ 2 {\text{IrCl}}_{6}^{2 - } + 2{\text{OH}}^{ - } \rightleftharpoons 2 {\text{IrCl}}_{6}^{3 - } + 0.5{\text{O}}_{2} + {\text{H}}_{2} {\text{O}} $$
(2)
Therefore, \( {\text{IrCl}}_{6}^{2 - } \) is rapidly reduced to \( {\text{IrCl}}_{6}^{3 - } \) by potassium iodide or sodium oxalate when the solution pH is higher than 11 [43]. A variety of organic compounds can be oxidized by \( {\text{IrCl}}_{6}^{2 - } \). With few exceptions, phosphorus, arsenic and sulfur donors can reduce Ir(IV) to Ir(III) [8]. The presence of various Ir(IV) and Ir(III) species in acidic chloride depends on the pH and chloride concentration. A scheme on the formation and conversion of the various species from the iridium(IV)–perchloric acid–chloride system is shown in Fig. 2 [50].
Fig. 2

Reaction scheme for the system Ir(III), Ir(IV) in acid solution [50]

3.3 Rhodium(III) Species

In aqueous solution, rhodium exists as the stable yellow hexaqua ion \( {\text{Rh(H}}_{ 2} {\text{O)}}_{6}^{3 + } \). It can be prepared by dissolving R2O3 in inorganic acids. The hydration number of rhodium has been confirmed as 5.9 ± 0.4 [43]. The hydroxy species Rh(H2O)5(OH)2+ exists when the acid concentration is less than 0.1 mol·L−1 and its pKa value is approximately 3.3. When an aqueous solution containing \( {\text{Rh(H}}_{2} {\text{O)}}_{6}^{3 + } \) is heated in excess HCl, rose-pink hexachlororhodate \( {\text{RhCl}}_{6}^{3 - } \) is formed. The hydrated chloride salt RhCl3·nH2O (n = 3, 4) is the starting material for the preparation of rhodium complexes. This compound can be obtained by dissolving hydrous Rh2O3 in aqueous hydrochloric acid and evaporating the hot solution [43]. Rhodium has seven species of aqueous and chloro-complexes: \( {\text{Rh(H}}_{2} {\text{O)}}_{6}^{3 + } \), \( {\text{RhCl(H}}_{2} {\text{O)}}_{5}^{2 + } \), \( {\text{RhCl}}_{2} ( {\text{H}}_{2} {\text{O)}}_{4}^{ + } \), RhCl3(H2O)3, \( {\text{RhCl}}_{4} ( {\text{H}}_{2} {\text{O)}}_{2}^{ - } \), \( {\text{RhCl}}_{5} ( {\text{H}}_{2} {\text{O)}}^{2 - } \) and \( {\text{RhCl}}_{6}^{3 - } \) [20]. A speciation diagram for the seven complexes of rhodium is shown in Fig. 3 [51, 52]. When \( {\text{Rh(H}}_{2} {\text{O)}}_{6}^{3 - } \) is heated in dilute hydrochloric acid, it forms different cationic species such as \( {\text{RhCl(H}}_{2} {\text{O)}}_{5}^{2 + } \) and \( {\text{RhCl}}_{2} ({\text{H}}_{2} {\text{O}})_{4}^{ + } \). As the acid concentration increases, \( {\text{RhCl}}_{6}^{3 - } \) becomes predominant [53]. In aged solutions of high chloride concentration, the two main rhodium species are \( {\text{RhCl}}_{6}^{3 - } \) and RhCl5(H2O)2− [54, 55]. With the passage of time, these species become hydrated and change their forms to several aqua-chloro complexes [56]. The steric effect of the reaction is governed by the trans effect of chloride in solution [57, 58].
Fig. 3

Distribution of Rh(III) chloride complexes with HCl concentration at 0.001 mol·L−1 RhCl3 as reported in the literature [52]

4 Separation of Iridium(IV) and Rhodium(III) by Solvent Extraction

In the separation of Ir(IV) and Rh(III) by solvent extraction, various extractants are employed. Therefore, the solvent extraction system will be reviewed according to the type of extractant, like amine, neutral and cationic extractants. The chemical formulae as well as the applications in the separation of iridium and rhodium of some common extractants are represented in Tables 4 and 5, respectively. Table 5 indicates that the two-metal separation systems can be categorized into three types depending on the separation purpose, namely hydrometallurgical applications, analytical purposes or both. Most separation systems focus on the selective separation of iridium and rhodium for the purification of precious metals [1, 2, 52, 59, 60, 61, 62, 63, 64]. A small number of works pay attention to the quantitative analysis of the composition of metals as well as their physical and chemical properties [65] or to the extraction mechanism by extractant mixtures [66]. In some separation systems, hydrometallurgical as well as analytical purposes are fulfilled by selecting suitable extractants, leading to some advantage in process economics [67, 68, 69, 70, 71].
Table 4

Chemical formulae and structures of the extractants

R an alkyl groups, Ph a phenyl group

Table 5

A summary of application in the separation of iridium and rhodium by various extractants

Extractant

Organic phase

Application in

Ref.

LIX 63a

Kerosene

Hydrometallurgical

[64]

Polyurethanea

Hydrometallurgical

[63]

TBPb

Toluene

Hydrometallurgical

[1]

Toluene

Hydrometallurgical

[1]

Toluene

Hydrometallurgical

[52]

Hydrometallurgical

[82]

Hexane

Both*

[69]

TPPb

1,2-Dichloroethane

Analytical (identify mechanisms)

[66]

PSb

Xylene

Hydrometallurgical

[2]

Isopentyl alcoholb

Both*

[70]

Diantipyryl propylmethaneb

Dichloroethane

Analytical (quantitative)

[65]

Cyanex 921b

Kerosene

Hydrometallurgical

[61]

Alamine 308c

Kerosene

Hydrometallurgical

[59]

Aliquat 336c

Kerosene

Hydrometallurgical

[59]

TEHAc

Kerosene

Hydrometallurgical

[59]

TOAc

Kerosene

Hydrometallurgical

[59]

Benzene

Hydrometallurgical

[60]

Alamine 336c

Toluene

Hydrometallurgical

[1]

Toluene

Hydrometallurgical

[1]

Kerosene

Hydrometallurgical

[62]

Kerosene

Hydrometallurgical

[61]

Alamine 300c

Dodecane

Hydrometallurgical

[61]

NPc

Chloroform

Bothd

[71]

2-Mercaptobenzothiazolec

Chloroform

Bothd

[67]

Diphenylthioureac

Chloroform

Bothd

[68]

Aliquat 336c + LIX 54a

Dodecane

Hydrometallurgical

[61]

aAcidic extractants

bNeutral extractants

cAmine-based extractants

dBoth analytical and hydrometallurgical application

*Mean application in both analytical and hydrometallurgical

Besides the extractants, diluents also play a very important role in the separation of precious metals by solvent extraction. It is usually used to dilute the extractants and added as a modifier agent to prevent the formation of a third phase or emulsion [72]. Normally, diluents such as kerosene, toluene, xylene and benzene are used for the extraction of metals [1, 2, 5, 9, 60]. However, it should be mentioned that toluene and chloroform are not acceptable for real operation owing to their chemical hazards. In some cases, dichloroethane or chloroform is employed for analysis [65, 67, 68] (see Table 5).

The separation of iridium(IV) and rhodium(III) is very difficult due to their similar physical and chemical properties and becomes a major challenge for researchers. Therefore, many single extraction systems have been employed to separate iridium and rhodium. However, few works have been reported employing synergistic extraction systems. It has been reported that the mixture of Aliquat 336 and LIX 54 could improve the separation of Ir(IV) and Rh(III) [61]. However, the low extraction percentage of Ir(IV) and stripping efficiency of Rh(III) are problems in this extraction system. Some single extractants for the selective extraction of Ir(IV) over Rh(III) have been reported, while several synergistic extraction systems have been used to selectively extract Rh(III) [73, 74].

Ionic liquids are known as green chemicals and have become useful in the solvent extraction process. A new branch of extractive metallurgy called solvometallurgy has been named to describe the processes in which nonaqueous solutions are employed instead of water [75]. Some studies have been reported on the use of ionic liquids for the extraction of either Ir(IV) or Rh(III) [76, 77, 78]. However, to our knowledge, the application of ionic liquids to the separation of Ir(IV) and Rh(III) is still limited. Therefore, we only focus on separating the two metals by solvent extraction with commercial extractants.

4.1 Separation Using Amines

Amines are the most common extractants for the separation of iridium and rhodium from acidic or alkaline solutions due to the difference in the formation of different metal ion species. The extraction and separation of Ir(IV) and Rh(III) from synthetic chloride solutions was investigated by using amine extractants (TOA, TEHA, Alamine 308, and Aliquat 336) in the HCl concentration range from 1 to 8 mol·L−1 [59, 79]. In this range of experimental conditions, Ir(IV) was selectively extracted over Rh(III) by these extractants. There was not much difference in the extraction percentages of these metals with changes in HCl concentration. Although the extraction efficiency of Ir(IV) from chloride solutions obtained with amines is high, the main drawback is high co-extraction of rhodium, resulting in a low separation factor of the two metals. Moreover, highly viscous amines (e.g., Aliquat 336) cause some difficulties in the continuous process for the extraction and stripping of metals.

In solvent extraction, the separation factor is defined as the ratio of the distribution coefficient of a target metal ion to that of another metal ion. A higher separation factor indicates that a target metal ion can be selectively extracted over other impurity metal ions by the extractant. On the basis of the separation factor and stripping, Aliquat 336 was found to be more effective than other amines. The highest separation factor of 82.5 between iridium and rhodium was obtained from 1 mol·L−1 HCl solution with Aliquat 336. Since \( {\text{RhCl}}_{6}^{3 - } \) predominates in the HCl concentration range of 1–8 mol·L−1 [1], the extraction reaction of Ir(IV) by Aliquat 336 and protonated tertiary amines can be represented as Eqs. 3 and 4:
$$ 2 {\text{R}}_{4} {\text{NCl}}_{\text{org}} + {\text{RhCl}}_{{6,{\text{aq}}}}^{3 - } \rightleftharpoons ({\text{R}}_{4} {\text{N}})_{ 2} {\text{IrCl}}_{{6,{\text{org}}}} + 2{\text{Cl}}_{\text{aq}}^{ - } $$
(3)
$$ 2 {\text{R}}_{3} {\text{NHCl}}_{\text{org}} + {\text{RhCl}}_{{6,{\text{aq}}}}^{3 - } \rightleftharpoons ({\text{R}}_{3} {\text{N}})_{ 2} {\text{IrCl}}_{{6,{\text{org}}}} + 2{\text{Cl}}_{\text{aq}}^{ - } $$
(4)

In the solvent extraction of metal ions, an unexpected phenomenon is often observed when the aqueous and organic phases are mixed together. The organic phase splits into two layers: a light phase and a heavy organic phase which is called the third phase [72]. The addition of TBP to Aliquat 336 prevents the formation of a third phase [1]. The extraction of Rh(III) was found to decrease from 64 to 22% with solution aging up to 2 weeks. However, aging did not show any effect on the extraction efficiency of Ir(IV). Among the stripping reagents such as NH4Cl, (NH2)2CS, (NH2)2CS + HCl, NaCl, NaOH, Na2CO3, and HClO4, HClO4 solutions could strip the two metal ions well from the loaded organic and the stripping percentage was proportional to HClO4 concentration. However, a mixture of (NH2)2CS and HCl could selectively strip Ir(IV) over Rh(III) from the loaded organic phase, indicating a possibility for separation of the two metal ion during the stripping. The stripping percentages of Ir(IV) and Rh(III) were 90.5 and 39.7%, respectively. The concentration of HCl in the mixture had a greater influence than that of thiourea on the stripping of Ir(IV) [59].

The effect of SnCl2 on the separation of Ir(IV) and Rh(III) using Alamine 336 has been reported [1, 52]. In the absence of SnCl2, the extraction percentage of Ir(IV) by Alamine 336 was much higher than that of Rh(III) from the mixed solution. The highest separation factor was obtained in 9 mol·L−1 HCl. Extraction reactions of Ir(IV) and Rh(III) by Alamine 336 from HCl solution can be described as:
$$ {\text{R}}_{3} {\text{NHCl}}_{\text{org}} + {\text{IRCl}}_{{5,{\text{aq}}}}^{ - } \rightleftharpoons {\text{R}}_{3} {\text{NHIrCl}}_{{5,{\text{org}}}} + {\text{Cl}}_{\text{aq}}^{ - } $$
(5)
$$ 2 {\text{R}}_{3} {\text{NHCl}}_{\text{org}} + {\text{RhCl}}_{5} ({\text{H}}_{2} {\text{O}})_{\text{aq}}^{2 - } \rightleftharpoons ({\text{R}}_{3} {\text{NH}})_{ 2} {\text{RhCl}}_{5} ({\text{H}}_{2} {\text{O}})_{\text{org}} + 2{\text{Cl}}_{\text{aq}}^{ - } $$
(6)
$$ 3 {\text{R}}_{3} {\text{NHCl}}_{\text{org}} + {\text{RhCl}}_{{6,{\text{aq}}}}^{3 - } \rightleftharpoons ({\text{R}}_{3} {\text{NH}})_{ 3} {\text{RhCl}}_{{6,{\text{org}}}} + 3{\text{Cl}}_{\text{aq}}^{ - } $$
(7)
However, adding SnCl2 to the mixed solution depressed the extraction of Ir(IV), while the extraction of Rh(III) was increased owing to the reducing action of SnCl2 [1]. The Sn(II) chloride in the solution reduced Rh(III) to Rh(I) according to Eqs. 8 and 9. The highest separation factor between Rh and Ir was around 1750 in the presence of SnCl2.
$$ {\text{RhCl}}_{6}^{3 - } + 6 {\text{SnCl}}_{3}^{ - } \rightleftharpoons {\text{Rh(SnCl}}_{3} )_{5}^{4 - } + {\text{SnCl}}_{6}^{2 - } + 3{\text{Cl}}^{ - } $$
(8)
$$ {\text{RhCl}}_{5} ({\text{H}}_{2} {\text{O}})^{2 - } + 12{\text{SnCl}}_{3}^{ - } \rightleftharpoons {\text{Rh(SnCl}}_{3} )_{5}^{4 - } + {\text{SnCl}}_{6}^{2 - } + 6{\text{SnCl}}_{3}^{ - } + 2{\text{Cl}}^{ - } {\text{ + H}}_{2} {\text{O}} $$
(9)

The formation of a third phase was overcome by adding either decanol or TBP to Alamine 336 [61, 62]. It has been reported that decanol had a negative effect on the extraction of Ir(IV) and thus TBP was recommended as a modifier [61]. The Ir(IV) in the loaded Alamine 336 solution was successfully stripped using Na2CO3 solution or a mixture of NaOH and NaCl [62]. The mixture of NaOH and NaCl can also be employed to strip Ir(IV) from the loaded Alamine 300 [61]. The disadvantage of these systems is the large consumption of chemicals. The selective extraction of Ir(IV) over Rh(III) by tri-n-octylamine in benzene was reported and it was confirmed that an anion exchange mechanism is responsible for the extraction [60]. More than 98% of Ir(IV) in the organic phase was stripped with ammonia solution. However, it is possible to reduce Ir(IV) to Ir(III) in the presence of chloride during this process. The Rh(III) ions were selectively extracted over Ir(IV) from 4 to 6 mol·L−1 hydrochloric acid solution by using 2-mercaptobenzothiazole, with its anionic –SH group and the uncharged basic =N– group, in chloroform [67]. By applying this method, rhodium is readily precipitated from boiling solution, which can be employed to determine the rhodium content [80]. The use of di-phenyl-thiourea in the presence of tin(II) chloride led to selective extraction of Rh(III) over Ir(IV) [68].

4.2 Separation Using Neutral Extractants

Extraction of Ir(IV) and Rh(III) using neutral extractants, such as TBP [1, 81], TPP [66], TOPO [61], petroleum sulfoxides (PS) [2] and some antipyrine derivatives [65] have been reported. The separation of Ir(IV) and Rh(III) from a nitric acid–sodium nitrate mixture could be achieved with TBP by nine stages of counter-current extraction [82]. The successful operation of a nine-stage extraction indicates that the aging of the extractants does not degrade their stability and there is little problem in regeneration of the extractants. A process with TBP was reported for hydrochloric acid solution, which can be applied to the analysis of micro amounts of Rh(III) in the solution [83]. A scheme was proposed for the separation of iridium and rhodium from other PGM group elements using TBP [69]. In the solvent extraction system of iridium and rhodium with TBP, a high concentration of extractant is employed at high acidity, which has a negative effect on the environment as well as the economics of the process [1]. Extraction of Ir(IV) and Rh(III) at low HCl concentration by TBP led to low extraction percentage of both metals and thus addition of SnCl2 was used enhance the selective extraction of Rh(III) [52]. The added SnCl2 reduces Rh(III) and Ir(IV) to Rh(I) and Ir(III), respectively. The Rh(I) can be extracted by TBP, while Ir(III) is not extracted by TBP [81, 84, 85].

A solvation reaction is responsible for the extraction of metals from 8 mol·L−1 HCl by TPP in 1,2-dichloroethane in the presence of tin(II) chloride [66]. However, there are no data or information on the separation of these metal ions from the solution. Separation of Ir(IV) and Rh(III) from chloride solution by petroleum sulfoxides (PS) was investigated and the extraction of Ir(IV) was proportional to the HCl concentration [2]. The separation factor increased with the increase in molar ratio of Rh(III) to Ir(IV), indicating that a small amount of Ir(IV) can be selectively extracted from a large amount of Rh(III) in the feed. The Ir(IV) in the loaded organic phase was completely stripped by using sodium hydroxide. The stripping percentage of Ir(IV) was as high as 99.5% and solvent extraction reactions were identified to be as follows [28, 86]:
$$ {\text{IrCl}}_{{ 6 , {\text{aq}}}}^{2 - } + 2{\text{H}}_{\text{aq}}^{ + } + 2{\text{L}}_{\text{org}} \rightleftharpoons ({\text{HL}}^{ + } )_{2} ({\text{IrCl}}_{6}^{2 - } )_{\text{org}} $$
(10)
$$ [ {\text{RhCl}}_{5} ( {\text{H}}_{2} {\text{O)]}}_{\text{aq}}^{2 - } + {\text{H}}_{\text{aq}}^{ + } + 2{\text{L}}_{\text{org}} \rightleftharpoons [({\text{LH}})]^{ + } [{\text{RhCl}}_{4} ({\text{H}}_{2} {\text{O}}){\text{L}}]_{\text{org}}^{ - } + {\text{Cl}}_{\text{aq}}^{ - } $$
(11)
where L is a petroleum sulfoxide molecule.
A solvent extraction system for the separation of Rh(III) and Ir(IV) using iso-pentyl alcohol with tin(II) bromide in perchloric acid medium was reported [70] and this system is suitable for the determination of micro amounts of rhodium and iridium [87]. Some anti-pyrine derivatives, such as di-anti-pyryl-methane, di-anti-pyryl-propyl-methane and di-anti-pyryl-phenyl-methane have been employed for the separation of iridium and rhodium from the other platinum group metals [65]. In acidic solutions, these reagents are present as cations and react with the anionic metal complexes as follows, where R stands for anti-pyrine, X is a halide ion and M is a platinum group metal:
$$ {\text{R}}_{\text{org}} + {\text{H}}_{\text{aq}}^{ + } \rightleftharpoons {\text{RH}}_{\text{org}}^{ + } $$
(12)
$$ [ {\text{M}}^{m} {\text{X}}^{n} ]_{\text{aq}}^{{(m{-}n)}} + (m + n){\text{RH}}_{\text{org}}^{ + } \rightleftharpoons ({\text{RH}})_{{(m{-}n)}} \left[ {{\text{M}}^{m} {\text{X}}_{n} } \right]_{\text{org}} $$
(13)

Electrostatic force as well as hydrogen bonding is responsible for the interaction between the cationic anti-pyrine and an anion [88]. From this system, Ir(IV) was selectively extracted over Rh(III). The separation of Ir(IV) and Rh(III) from a mixture of Ir–Ru–Rh using Cyanex 921 was investigated [61]. Cyanex 921 can selectively extract Ir(IV) from a 1 mol·L−1 HCl solution. The loaded Ir(IV) solution was successfully stripped by distilled water.

4.3 Separation Using Cationic Extractants

Since Ir(IV) and Rh(III) predominantly exist as anionic complexes in hydrochloric acid solutions, the use of cationic extractants for the separation of these two metals has been reported [37, 45, 46, 47, 49, 50, 51, 52, 55, 56, 57, 89, 90, 91]. Separation of Ir(IV) and Rh(III) with polyether-type polyurethane foam from hydrochloric acid solution was investigated in the presence of LiCl and thiocyanate [92]. Although the basic function of polyurethane foam is to adsorb metal ions, the above-mentioned polyurethane foam system study focused on the effect of the presence of some salts such as LiCl, NaCl, and KCl in the foam. The presence of these salts in the foam altered the mechanism of metal extraction from adsorption to solvent extraction. Selective extraction of Rh(III) from solution of 0.01–5 mol·L−1 HCl was achieved with a polyether-type polyurethane foam [63]. The extraction reaction of anionic metal complexes (\( {\text{MeX}}_{n}^{m - } \)) by the foam can be represented by Eq. 14 [93]:
$$ m{\text{M}}^{p + } + p{\text{MeX}}_{n}^{m - } + m\overline{{{\text{site}}}} \rightleftharpoons \overline{m({\text{M}} {\cdot}{\text{site}}})^{p + } + p\overline{{{\text{MeX}}_{n}^{m - } }} $$
(14)
where Mp+ represents cations (such as Li+, Na+, \( {\text{NH}}_{4}^{ + } \), and H3O+) that can be multiply complexed by the polymer at specific sites. The bar represents anionic metal complexes (e.g., \( {\text{Rh}}({\text{SCN}})_{6}^{3 - } \)) associated within the polymer phase.
In the presence of thiourea, HDEHP selectively extracted rhodium over iridium from acidic solution [94]. A process using LIX 63 was developed to separate Pd(II), Pt(IV), Ir(IV) and Rh(III) from concentrated hydrochloric acid solutions [64, 95]. While LIX 63 selectively extracted Pd(II) from the four different types of metal ions in HCl solutions of concentrations 1–6 mol·L−1, Pt(IV) in the raffinate was selectively extracted by TBP. Since a fraction of Ir(IV) was reduced to Ir(III), the iridium and Rh(III) in the raffinate was separated by Aliquat 336 after adding NaClO3 as an oxidizing agent. The disadvantage of this process is the consumption of expensive chemicals. The Ir(IV) in the loaded Aliquat 336 phase was easily stripped by HClO4 solution. Although reuse of the extractants was not mentioned, it is possible to regenerate the loaded Aliquat 336 by washing with water. A summary of the separation of Ir(IV) and Rh(III) including extraction and stripping conditions from different solutions by various extractants is presented in Table 6.
Table 6

Summary of separation of iridium(IV) and rhodium(III) from different solution by various extractants

Extractant

Organic phase

Aqueous

Selectivity

Strippant

SF

Ref.

LIX 63a

Kerosene

[Ir] = [Rh] = [Pd] = [Pt] = 100 mg·L−1, [HCl] = 6 mol·L−1, [NaClO3] = 0.001 mol·L−1

Ir over Rh

HClO4

[64]

Polyurethanea

[Ir] = [Rh] = 15 mg·L−1, [HCl] = 2 mol·L−1, 0.002 mol·L−1 thiocyanate

Rh over Ir

70.8

[63]

TBPb

Toluene

[Ir] = 0.00025 mol·L−1, [Rh] = 0.0005 mol·L−1, [HCl] = 8 mol·L−1

Ir over Rh

72

[1]

Toluene

[Ir] = 0.00025 mol·L−1, [Rh] = 0.0005 mol·L−1, [HCl] = 7 mol·L−1, [SnCl2] = 0.005 mol·L−1

Rh over Ir

365

[1]

Toluene

[Ir] = 0.0002 mol·L−1, [Rh] = 0.001 mol·L−1, [HCl] = 1 mol·L−1

Ir over Rh

<1

[52]

[Ir] = 177 mg·L−1, [Rh] = 247 mg·L−1, [HCl] = 5 mol·L−1 saturated with NaCl

Ir over Rh

25% HNO3 + NaNO3,

 %SIr = 94%

 %SRh = 99%

12.2

[82]

Hexane

[Ir] = 106 mg·L−1, [Rh] = 51 mg·L−1, [HCl] = 6 mol·L−1

Ir over Rh

10% HBr

 %SIr = 97%

2213

[69]

TPPb

1,2-Dichloroethane

[Ir] = [Rh] = 0.005 mol·L−1, [HCl] = 6 mol·L−1

Ir over Rh

[66]

PSb

Xylene

[Ir] = [Rh] = 100 mg·L−1, [HCl] = 4 mol·L−1

Ir over Rh

0.3%wt NaOH

 %SIr = 99.5%

1450

[2]

Isopentyl alcoholb

[Ir] = 504 mg·L−1, [Rh] = 732 mg·L−1, [HClO4] = 4.5 mol·L−1

Ir over Rh

1.5 mol·L−1 HBr

 %SIr = 99%

 %SRh = 98%

[70]

Diantipyryl

propylmethaneb

Dichloroethane

[Ir] = 0.8 g·L−1, [Rh] = 1 g·L−1,

[HCl] = 1 mol·L−1

Ir over Rh

357

[65]

Cyanex 921b

Kerosene

[Ir] = [Ru] = [Rh] = 50 mg·L−1, [HCl] = 1 mol·L−1, NaCl = 4 mol·L−1

Ir over Rh

H2O

 %SIr = 84%

 %SRh = 0%

453

[61]

Alamine 308c

Kerosene

[Ir] = 200 mg·L−1, [Rh] = 120 mg·L−1, [HCl] = 1 mol·L−1

Ir over Rh

55.1

[59]

Aliquat 336c

Kerosene

[Ir] = 200 mg·L−1,

[Rh] = 120 mg·L−1, [HCl] = 1 mol·L−1

Ir over Rh

1 mol·L−1 (NH2)2CS + 3 mol·L−1 HCl

 %SIr = 90.5%

 %SRh = 39.7%

82.5

[59]

TEHAc

Kerosene

[Ir] = 200 mg·L−1, [Rh] = 120 mg·L−1, [HCl] = 1 mol·L−1

Ir over Rh

76.6

[59]

TOAc

Kerosene

[Ir] = 200 mg·L−1, [Rh] = 120 mg·L−1, [HCl] = 1 mol·L−1

Ir over Rh

77.1

[59]

 

Benzene

[Ir] = 1.4 g·L−1, [Rh] = 1.5 g·L−1, [HCl] = 6 mol·L−1

Ir over Rh

7 mol·L−1 NH4OH

 %SIr = 98%

 %SRh = 96%

1427

[60]

Alamine 336c

Toluene

[Ir] = 0.00025 mol·L−1, [Rh] = 0.0005 mol·L−1, [HCl] = 9 mol·L−1

Ir over Rh

25.2

[1]

 

Toluene

[Ir] = 0.00025 mol·L−1,

[Rh] = 0.0005 mol·L−1, [HCl] = 9 mol·L−1, [SnCl2] = 0.01 mol·L−1

Rh over Ir

1750

[1]

 

Kerosene

[Ir] = [Ru] = [Rh] = 100 mg·L−1, [HCl] = 1 mol·L−1

Ir over Rh

1 mol·L−1 Na2CO3

 %SIr =  %SRh = 100%

9

[62]

 

Kerosene

[Ir] = [Ru] = [Rh] = 50 mg·L−1, [HCl] = 1 mol·L−1, NaCl = 1 mol·L−1

Ir over Rh

0.2 mol·L−1 NaOH + 0.35 mol·L−1 NaCl

 %SIr = 83%

 %SRh = 57%

17.3

[61]

Alamine 300c

Dodecane

Ir = Ru = Rh = 50 mg·L−1, [HCl] = 1 mol·L−1, NaCl = 1 mol·L−1

Ir over Rh

0.2 mol·L−1 NaOH + 0.35 mol·L−1 NaCl

 %SIr = 92%

 %SRh = 52%

15.1

[61]

NPc

Chloroform

[Ir] = [Rh] = 0.0005 mol·L−1, [HCl] = 0.08 mol·L−1,

[Cl] = 3.7 mol·L−1

Co-extraction

[71]

2-Mercaptobenzothiazolec

Chloroform

[Ir] = [Rh] = 1 g·L−1,

[HCl] = 4–6 mol·L−1

Rh over Ir

[67]

Diphenylthioureac

Chloroform

[Ir] = [Rh] = 1 g·L−1,

[HCl] = 1–2 mol·L−1

Ir over Rh

[68]

Aliquat 336c + LIX 54a

Dodecane

[Ir] = [Ru] = [Rh] = 50 mg·L−1, [HCl] = 1 mol·L−1, NaCl = 4 mol·L−1

Ir over Rh

2 mol·L−1 NH3 + 3 mol·L−1 NH4Cl

 %SIr = 100%

 %SRh = 0%

35.5

[61]

SF separation factor,  %S percentage of stripping

aAcidic extractants

bNeutral extractants

cAmine-based extractants

5 Conclusions

The properties, sources and applications of iridium and rhodium together with the separation of Ir(IV) and Rh(III) using solvent extraction were reviewed and the production processes of pure iridium and rhodium metal in the recycling industry were considered. Since the oxidation states of rhodium and iridium are important in the solvent extraction process, the aqueous chemistry of Ir(IV) and Rh(III) was discussed. For the separation of Ir(IV) and Rh(III), various extractants, such as amine, neutral and cationic extractants have been used. The most suitable conditions for each type of extractant were reviewed. Amines are promising extractants for the separation of Ir(IV) and Rh(III) when the HCl concentration is not high. By contrast, Ir(IV) and Rh(III) are well extracted by neutral extractants at high acid concentration. In comparison to amines and neutral extractants, the use of cation extractants for the separation of iridium and rhodium has been reported. Selective extraction of Ir(IV) over Rh(III) has been demonstrated in most of the separation systems owing to the difference in charge density of the anionic species of Ir(IV) and Rh(III). Rhodium can be selectively extracted over iridium by neutral extractants in the presence of a reducing agent like tin(II) chloride.

Further work should be made to develop an efficient process to separate Ir(IV) and Rh(III) by solvent extraction. Changing the oxidation state of the metals is one way to find efficient extraction and separation systems. Since inorganic reducing agents have some effect on the extraction of the metals, it is better to research organic reducing agents that would not affect the purity of the metals thus extracted. Another alternative for the selective extraction of one metal over other metal is to use masking agents such as citrate, malonate, and oxalate. Ionic liquids have been employed on the extraction and recovery of metals and thus some additional work can be done here to improve the separation efficiency of Ir(IV) and Rh(III).

Notes

Acknowledgements

This work was supported by the Global Excellent Technology Innovation of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20165010100810). We also thank Murdoch University for collaboration opportunities.

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© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Advanced Materials Science & Engineering, Institute of Rare MetalMokpo National UniversityMokpoKorea
  2. 2.Chemical & Metallurgical Engineering & Chemistry, School of Engineering and Information TechnologyMurdoch UniversityMurdoch, PerthAustralia

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